Dispatch calling originally was used in fleet radio systems where a central dispatcher would communicate with remotely located mobile radios, such as in trucking operations, and public safety radio systems. Today this concept has evolved into a type of call between two or mote parties, and where the communication is essentially simplex or half-duplex in nature, and where the call is carried in part over a fixed equipment network. This type of calling has become very popular with small businesses and even between family members. An example of a present-day state of the art dispatch system is that operated by Nextel Communications, Inc. and which uses iDEN mobile phones and infrastructure equipment manufactured by Motorola, Inc. Indeed, the so-called “private call” feature of these systems have become a key marketing factor.
In general, the private call is a dispatch call between two parties. A first party knows the private identifier of the second party, and uses that to make a dispatch call to the second party. The advantage of a dispatch call is the speed with which the other party can be contacted, and the “caller speaks first” aspect of a dispatch call. That is, when the first party initiates a dispatch call to the second party, the communication system rapidly sets up the call, and sends a message to the first party's radio or mobile communication device. Upon receiving the message, the first party's radio generates an audible alert to let the first party know when to begin speaking. The second party's radio receives the voice signal and plays it for the second party to hear, with the second party having to take any action. The speed with which one party can communicate information to another party is significantly less compared to setting up a mobile telephone call between the tow radios.
Presently commercially available dispatch service is done with time division multiple access (TDMA) air interfaces. TDMA schemes assign each radio to a frequency and a specific time slot in a repeating time frame. While TDMA achieves higher capacity of traffic in a given frequency spectrum over systems that are merely frequency division multiple access (FDMA), code division multiple access (CDMA) systems permit even greater capacity. However, CDMA systems present a substantial problem in implementing dispatch calling.
In a CDMA system several mobile communication devices communicate on the same frequency at the same time, but use different pseudorandom codes to define different channels. CDMA systems achieve higher capacity through extensive management of each channel, and in particular managing the power at which the mobile communication device transmits so that other mobile communication devices will not have their signal overwhelmed or otherwise corrupted. However, power management in a CDMA air interface contributes significantly to the delay in setting up a call, and has been a prohibiting factor in designing a CDMA system that can set up calls rapidly, as is necessary to implement dispatch calling. Another source of delay in presently available CDMA systems is that standard telephony switching is used which requires a call to first be set up between the mobile communication device and the fixed equipment network, which then switches the call to the party being called who must answer the call before the calling party can speak. If the calling party is calling another CDMA service subscriber, then a link must be set up between the target party, and this is typically done after the link between the calling party has set up a link to the fixed equipment network. This delay results in an unacceptable amount of time being required for dispatch calling. Therefore there is a need for a method and a system which facilitates rapid call set up in a system using a CDMA air interface so that dispatch calling may be implemented.